This invention relates generally to gas turbine engines and, more particularly, to combustors adapted to minimize exhaust emissions.
A currently active area of gas turbine engine technology is exhaust emission control in automotive gas turbine applications. In particular, control of hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NO.sub.x) is essential. HC and CO can be held to acceptable concentrations by efficient and complete combustion. In simple diffusion flame combustors, where fuel is sprayed directly into the combustion or primary reaction chamber and burns essentially at the stoichiometric air-fuel ratio, HC and CO do not present problems. Stoichiometric combustion, however, results in maximum flame temperature and maximum NO.sub.x formation. To achieve lower NO.sub.x levels than those of the simple diffusion flame combustors, other combustor designs have been proposed wherein a lean fuel-air mixture burns completely but at a lower temperature where the level of NO.sub.x in the exhaust is acceptable. In this type of combustor, known as a prevaporization combustor, a lean fuel-air mixture is prepared in a premixing-prevaporization chamber, called a prechamber, wherein the amount of air entering the chamber is closely controlled in relation to the amount of fuel injected into the prechamber to maintain a desired fuel-air ratio. The fuel injected into the prechamber vaporizes in the controlled quantity of air and the lean mixture is conveyed into the primary reaction chamber of the combustor where it burns at the low temperature. While such solutions are known to be effective in NO.sub.x control, the combustor structure is complicated by the fact that the quantities of air and fuel admitted to the prechamber must be changed frequently because of the variable nature of the automotive operating cycle. In some heretofore proposed prevaporization combustors, systems of movable shutters and/or louvers have been provided on the prechamber and on the combustor to control the amount of air admitted to the prechamber and to divert more air from the primary reaction chamber to the prechamber at higher engine operating speeds. While such systems effectively control emissions they also develop undesirably high pressure drops across the combustor at high engine power levels and also introduce complicated control mechanisms not particularly suited to severe gas turbine combustor operating conditions. A gas turbine engine combustor according to this invention is an improvement over these and other heretofore known prevaporization type low emission combustors.